The present invention relates to a thin film resonator and a method for manufacturing the same, and more particularly to an integrated thin film resonator with multiple bands to enhance performance and an easy method for manufacturing the same.
Mobile communications has been rapidly developed as the main instrument serving the information society. This instrument has been influenced by the developments of two technologies: signal processing using modulation or demodulation of transmitted data over limited frequency bands, and the technology of manufacturing radio frequency (RF) hardware parts.
In particular, filters are most important among the parts used for RF mobile communication devices. Filters are able to select the signal requested by the user from numerous signals on the public communication network, or filter a signal transferred by the user. Thus, excellent filters were previously developed for high quality mobile communication. Recently, higher performance filters have been developed to be thinner and more light-weight. These features ensure that mobile communication devices consume less power and more portable.
In general, a resonator or a frequency filter is the device that transmits the frequencies of a message in a predetermined band, and filters the frequencies on other bands being produced by various electronic devices such as wireless phones, personal communication service devices, cellular phones or devices for the international mobile telecommunications 2000 (IMT-2000) as a band pass filter.
Presently, the dielectric filter and the surface acoustic wave (SAW) filter are widely used as the RF filter for mobile communication devices. The dielectric filter has some advantages such as high permeability, low insertion loss, stability at high temperatures and good mechanical strength. However, the dielectric filter is too large to be applied in a monolithic microwave integrated circuit (MMIC). Though the mono-blocked or the multi-layered surface mounted device (SMD) resonators are now developed with smaller dimensions, SMD resonators do not sufficiently overcome their size problems.
SAW filters are relatively smaller than dielectric filters and have simpler signal processing and more simplified circuits. The SAW filter also can be manufactured using semiconductor technology, and gives high quality results since the SAW filter""s side rejection in its pass-band is greater than that of the dielectric filter. However, the SAW filter has large insertion losses below 3 dB and its manufacturing costs are high because it is manufactured using single crystal piezoelectric substrate composed of lithium niobate (LiNbO3) or lithium titanate (LiTaO3). Also, the SAW filter is manufactured using an ultraviolet ray exposure apparatus so that the SAW filter may not be used for high frequency bands above 5 GHz because the line width of the inter-digital transducer (IDT) is above 0.5 xcexcm.
Film bulk acoustic resonators (FBAR) have been developed for next generation mobile communication devices. The FBAR can be mass-produced at low cost using semiconductor technology and is ultra light weight and thin. In addition, the FBAR can be freely combined with RF active devices. In particular, the FBAR has good insertion loss of about 1 to 1.5 dB-smaller than or identical to that of the dielectric filter. The FBAR also has excellent side rejection higher than the of the SAW filter by about 10 to 20 dB, thereby providing high quality results.
At present, the active elements of mobile communications include the Hetero-junction Bipolar Transistor (HBT) or the Metal Semiconductor Filed Effect Transistor (MESFET), but these are gradually being simplified and minimized by monolithic microwave integrated circuit (MMIC) technology. However, passive components of RF technology such as the filter, the duplexer filter or the antenna are relatively large and complicated structures so that the single chip may not be achieved due to passive components.
The FBAR or the stacked thin film bulk wave acoustic resonators (SBAR) are manufactured by forming piezoelectric material such as zinc oxide (ZnO) or aluminum nitride (AIN) on a substrate composed of silicon or gallium-arsenic (Gaxe2x80x94As) using RF sputtering method, thereby achieving the desired resonance provided by the piezoelectric material.
The thin film resonator can be manufactured at low cost and yet provide high quality, making it is applicable for use in various devices with frequency bands of 900 MHz to 10 GHz. In addition, the thin film resonator can be much smaller than the dielectric filter and has the added benefit of an insertion loss smaller than that of the SAW filter. Hence, a thin film filter such as FBAR can be used in any MMIC dependent on high quality and good stability.
The method for manufacturing conventional FBAR or the SBAR is disclosed at U.S. Pat. No. 6,060,818 issued to Richard C. Ruby et al.
FIG. 1 is a cross-sectional view showing the FBAR and FIGS. 2A to 2C are cross-sectional views illustrating the method for manufacturing the FBAR in FIG. 1.
Referring to FIG. 1, the FBAR 10 is formed on a silicon substrate 15 and the FBAR 10 includes a bottom electrode 20, a piezoelectric layer 25 and a top electrode 30.
An oxide layer 35 is formed on the substrate 15 and a pit 40 is interposed between the substrate 15 and the FBAR 10.
Referring to FIG. 2A, the silicon substrate 15 is provided, and then the pit 40 having a predetermined depth is formed on the substrate 15 by partially etching the substrate 15. Subsequently, the oxide layer 35 is formed on the whole surface of the substrate 15 by the thermal oxidation method.
As shown in FIG. 2B, after a sacrificial layer 45 composed of phosphor silicate glass is coated on the oxide layer 35 to fill pit 40, the sacrificial layer is polished so that the sacrificial layer 45 remains only in the pit 40.
Referring to, FIG. 2C, after the bottom electrode 20 composed of molybdenum (Mo), the piezoelectric layer 25 composed of aluminum nitride (AIN), and the top electrode 30 composed of molybdenum are successively coated on the oxide layer 35 and on the sacrificial layer 45 filling the pit 40, the bottom electrode 20, the piezoelectric layer 25, and the top electrode 30 are patterned. Then, the sacrificial layer 45 is removed using an etching solution containing hydrofluoric acid (HF), thereby completing the FBAR 10 as shown in FIG. 1.
The conventional FBAR is, however, formed on the substrate where the cavity is positioned, giving the FBAR two-dimensional construction. Hence, the conventional FBAR provides poor quality performance with an increased insertion loss.
In addition, the interference of the substrate may not be blocked, causing the power loss of the FBAR to increase. The size of the FBAR is limited also, since the FBAR is formed over the cavity in the substrate in order to receive the deformation of the piezoelectric layer.
Furthermore, the process for etching the silicon substrate demands much time, and the cost of manufacturing the FBAR increases because the conventional FBAR is formed on the silicon substrate where the cavity is positioned.
To overcome such problems, research institutes at Berkeley and Michigan Universities have disclosed a thin film bulk acoustic resonator (TFBAR) with a three-dimensional structure on a substrate using the micro-electromechanical system (MEMS) technology. However, the TFBAR may not be mass-produced and packaging the TFBAR may be difficult since its structure is complicated and the integration device including the TFBAR is difficult.
The present invention is intended to overcome the disadvantages described above. Therefore, it is an object of the present invention to provide a thin film resonator having an ultra minute size in order to achieve high integration with MEMS technology, and a method for manufacturing the thin film resonator.
It is another object of the present invention to provide a thin film resonator manufactured to be a multiple frequency band integrated thin film resonator by controlling the thickness of piezoelectric layer thereof, and a method for manufacturing the thin film resonator.
It is still another object of the present invention to provide a thin film resonator having a three-dimensional, floating construction to minimize power loss due to interference from the substrate, corresponding in size to the size of the substrate, and a method for manufacturing the thin film resonator.
It is still another object of the present invention to provide a thin film resonator manufactured at low cost, yet giving high quality results, and a method for manufacturing the thin film resonator.
It is still another object of the present invention to provide a thin film resonator having minute patterns and a three-dimensional form in order to obtain high quality results with low insertion loss, and a method for manufacturing the thin film resonator.
To accomplish the objects of the present invention according to one aspect of the present invention, there is provided a thin film resonator for filtering the frequency of a predetermined band comprising a supporting means having a plurality of posts formed on a substrate and a supporting layer formed on the posts, a first electrode formed on the supporting means, a dielectric layer formed on the first electrode, and a second electrode formed on the dielectric layer.
There are four posts formed on the substrate so as to support the supporting layer, and the supporting layer has a plurality of openings formed adjacent to each post, respectively.
Preferably, the supporting layer, the first electrode, the dielectric layer and the second electrode are each shaped like rectangular plates that, in combination, create a pyramid shape.
The first and the second electrodes are composed of metals selected from the group consisting of platinum, tantalum, platinum-tantalum, gold, molybdenum and tungsten. The dielectric layer is composed of materials selected from the group consisting of barium titanate, zinc oxide, aluminum nitride, lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT) and lead magnesium niobate (PMN).
The thin film resonator further comprises a connecting means for connecting the second electrode to a circuit formed on the substrate. The connecting means has a central portion and lateral potions bent from the central portion so that the connecting means contacts the circuit and the second electrode. As a result, a first air gap is interposed between the substrate and the supporting means and a second air gap is interposed between the second electrode and the connecting means. In this case, the connecting means is composed of metals selected from the group consisting of platinum, tantalum, platinum-tantalum, gold, molybdenum and tungsten.
To accomplish the objects of the present invention according to another aspect of the present invention, there is provided a method for manufacturing a thin film resonator for filtering frequencies on a predetermined band, which comprises the steps of forming a first sacrificial layer on a substrate, partially etching the first sacrificial layer to expose portions of the substrate, forming a plurality of posts on the exposed portions of the substrate, forming a first layer on the posts and the first sacrificial layer, forming a first metal layer on the first layer, forming a second layer on the first metal layer, forming a second metal layer on the second layer, forming a first electrode, a dielectric layer and a second electrode by patterning the second metal layer, the second layer and the first metal layer, forming a supporting layer having a plurality of openings by patterning the first layer, and removing the first sacrificial layer through the openings.
The first sacrificial layer is composed of poly silicon and formed by a low pressure chemical vapor deposition method and the first sacrificial layer is partially etched by a photolithography method, a reactive ion etching method or an argon laser etching method.
The posts are created through forming a BPSG layer on the first sacrificial layer and the substrate using a low pressure chemical vapor deposition method at temperatures under about 500xc2x0 C., and polishing the BPSG layer to remove portions of the BPSG layer formed on the first sacrificial layer. At that time, the BPSG layer is polished by a chemical mechanical polishing method or an etch-back method.
The first layer is formed by a plasma enhanced chemical vapor deposition method or by using silicon oxide or phosphor oxide at temperatures from approximately 350 to 450xc2x0 C.
The first and the second electrodes are formed by using metals selected from the group consisting of platinum, tantalum, platinum-tantalum, gold, molybdenum and tungsten using a sputtering method or a chemical vapor deposition method.
The second layer is formed from piezoelectric material or electrostrictive material using a sol-gel method, a sputtering method, a spin coating method or a chemical vapor deposition method. The second layer is composed of materials selected from the group consisting of barium titanate, zinc oxide, aluminum nitride, lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT) and lead magnesium niobate (PMN). In this case, the second layer is heat treated by a rapid thermal annealing method for the phase transition of the second layer.
The first sacrificial layer is removed with xenon fluoride or bromine fluoride.
Preferably, the method for manufacturing the thin film resonator comprises the steps of forming a second sacrificial layer on the substrate and the second electrode, partially etching the second sacrificial layer to expose a portion of the second electrode and a circuit formed on the substrate, forming a connecting means for connecting the second electrode to the circuit, and removing the second sacrificial layer.
The second sacrificial layer is composed of poly silicon or photo resist and formed by a low pressure chemical vapor deposition method or a spin coating method.
Preferably, the surface of the second sacrificial layer is planarized using a chemical mechanical polishing method or an etch-back method.
The connecting means is formed from metals selected from the group consisting of platinum, tantalum, platinum-tantalum, gold, molybdenum and tungsten using a sputtering method or a chemical vapor deposition method.
The second sacrificial layer is removed with xenon fluoride, bromine fluoride, etching solution containing hydrofluoric acid, or by using an argon laser etching method. At that time, the steps for removing the first and the second sacrificial layers are simultaneously performed.
In general, the resonator for filtering frequencies on a band operates according to the principle of resonance created due to a bulk acoustic wave generated from the piezoelectric layer that lies between two electrodes. The process for manufacturing such a resonator generally consists of forming the piezoelectric film composed of zinc oxide (ZnO) or aluminum nitride (AIN) on a substrate composed of silicon or gallium-arsenic (Gaxe2x80x94As), and forming a membrane and electrodes.
In the resonator manufacturing process, the piezoelectric film is fixed to the electrode and the piezoelectric film is adequately thin and flat, and of adequately high density. According to the conventional method, after the P+ layer including a boron or silicon oxide layer, is formed on the silicon substrate by an ionic growth method, the bottom of the silicon substrate is anisotropically etched until the membrane forms a cavity formed in the substrate. Then, electrodes are formed on the membrane and the piezoelectric layer is interposed between the electrodes by using an RF magnetron sputtering method to form the thin film resonator. The piezoelectric materials used to form the piezoelectric layer requires a high specific resistance below 106 xcexa9cm with a standard deviation below 60, a large electromechanical coupling constant, and good cultivation. In addition, the piezoelectric material should have high breaking strength and quality reproduction results. However, the manufacturing process, including production of the abovementioned membrane products, experiences much failure because the membrane may be fractured when the thin film resonator is separated for packaging. Also, the thin film resonator may have low resonance characteristics because acoustic wave energy is lost due to the membrane. Recently, an air gap typed FBAR or a brag reflector typed FBAR has been used to reduce this loss of acoustic wave energy due to the membrane, and to simplify the resonator manufacturing process.
As for the air gap type of FBAR, after a sacrificial layer is formed on a silicon substrate using micro-machining technology, the air gap is formed at the point where the sacrificial layer is located. Hence, the manufacturing time and the generation of harmful gases can be reduced without using back-etching to form the membrane.
In the brag reflector typed FBAR, materials, each with different acoustic impedances, are alternatively formed on the silicon substrate to facilitate the brag reaction, thereby generating the resonance of the acoustic wave energies between electrodes. The brag reflector typed FBAR can be utilized as a ladder filter, a monolithic crystal filter, a stacked filter or a lattice filter can be a one chip type of thin film resonator. Such resonators may be manufactured quickly and have high mechanical strength, but their low electromechanical coupling constant is reduced by 30% when compared to the conventional FBAR.
According to the present invention, the thin film resonator is manufactured using MEMS technology without etching the substrate to have minute dimensions below hundreds of micrometers. Hence, the thin film resonator is exceptionally small and can be highly integrated onto the substrate. Also, the thickness of the dielectric layer of the thin film resonator can be adjusted to achieve the integration of multiple bands including radio frequency (RF), intermediate frequency (IF) and low frequency (LF) by controlling the thickness of the dielectric layer. Also, an inductor and a capacitor can be integrated.
In addition, yields can be increased and manufacturing costs can be greatly reduced since the thin film resonator can be manufactured without etching or machining the silicon substrate. Therefore, the manufacturing process of the present invention has excellent advantages during mass production, including simplicity and ease of packaging. Also, the thin film resonator of the present invention has a good quality factor of about 1000 to 10000 and a low insertion loss of under 2 dB, because the thin film resonator has minute patterns and a three-dimensional, floating construction, and is easily manufactured using the MEMS technology.
Furthermore, the thin film resonator of the present invention can minimize any interference due to its substrate, and has ideal dimensions because of its compact substrate, making the thin film resonator exceptionally small yet comprising three-dimensional, floating construction.